1. Field of the Invention
The present invention relates to a digital signal transmission apparatus, a digital signal transmission method, and a digital signal transmitter-receiver, and is suitable for applying to a case of transmitting a digital audio signal by, for example, a radio transmission system.
2. Description of the Related Art
As this type of audio signal transmission method, there is an infrared transmission method. The infrared transmission method generates a transmission audio signal by frequency-modulating an audio signal at the transmitting side and driving an infrared optical emitter based on the frequency-modulated signal. At the receiving side, the transmission audio signal is received by an infrared photodetector and thereafter demodulated.
Thereby, this type of audio signal transmission method requires no transmission line and makes it possible to simultaneously transmit a desired audio signal to a plurality of audio units. Therefore, the audio signal transmission method is used for a wireless headphone, a speaker system, and so on.
However, the existing infrared transmission method has a problem that an audio signal is easily deteriorated because it is analog-modulated and then transmitted.
To solve this problem, an audio signal transmission method of driving an infrared optical emitter based on a digital audio signal and thereby transmitting a high-tone-quality digital audio signal by an infrared transmission method is proposed by the applicant of this application and disclosed in the U.S. Pat. No. 5,394,259.
However, the audio signal transmission method of this type drives an infrared optical emitter by directly using a digital audio signal to be originally transmitted by a coaxial cable or optical fiber, or a digital audio signal processed through EFM (Eight to Fourteen Modulation). Therefore, a problem occurs that the frequency band of the digital audio signal thus obtained cannot follow the frequency allocation of sub-carriers related to infrared transmission specified in CP-1205 of Electronic Industries Association of Japan.
That is, CP-1205 specifies the frequency allocation (sub-carrier) shown in FIG. 1 so as to transmit a remote control signal within a band of 0.33 to 0.4 MHz, a conference system signal and an analog audio signal within a band of 0.4 to 1 MHz, various data values within a band of 1 to 2 MHz, a high-tone-quality audio signal within a band of 2 to 6 MHz, and a video signal within a band of 6 to 30 MHz.
Thus, a digital audio signal must be transmitted within a frequency band of 2 to 6 MHz. However, an existing digital audio signal is transmitted also by using a band other than the above frequency band.